The present invention relates generally to methods of permanently emplacing non-degradable, solid, colloidal materials into the subsurface in order to increase the retardation of a migrating contaminant plume, leading to improved plume management. More specifically, this invention relates to utilizing stabilized forms of colloidal sorbents that can be injected and distributed within the subsurface in order to engineer the effective hydrophobicity of the native aquifer and decrease the mobility of contaminant plumes.
Pollutant spills and releases that reach the subsurface, even in small quantities, can lead to extensive groundwater contamination. This contaminated water poses a serious human and ecological threat through various exposure pathways including coming in contact with potable groundwater supplies or by migrating under homes and businesses leading to vapor intrusion risks. Even if immediate contact with one of the exposure pathways does not occur, risk remains due the ability of the contaminant plume to migrate and expand, eventually coming into contact with a sensitive receptor.
Often times when contaminant plumes are identified, risk assessments will be performed to determine the action required to address the plume, which could range from active groundwater remediation to simply monitoring the natural attenuation of the plume. Fate and transport models are often used in the assessment to help to evaluate the human health or ecological threat. These models utilize the existing plume and aquifer characteristics including the contaminant concentration, the contaminant soil organic carbon-water partition constant (Koc), seepage velocity, natural biodegradation rates, the fraction of organic carbon (foc), and a calculated retardation factor to predict the extent of plume migration that can be expected over a certain time frame. In cases where plumes are found to be low-risk, a key aspect is often that the natural retardation provided by the native aquifer conditions for a given contaminant is large enough to prevent significant migration of the plume.
The amount of natural retardation that occurs within an aquifer varies directly with the levels of foc native to the aquifer as the foc provides the primary location for hydrophobic contaminants to partition and thereby prevent migration. This means that if the natural foc level is high enough in an aquifer, then a plume of a given contaminant of concern (COC) may be sufficiently retarded to effectively minimize the risk associated with migration. However, in many subsurface groundwater zones the foc is not substantial enough to increase the retardation and reduce the risk of a migrating contaminant plume. This can be especially true within the permeable channels of the subsurface that represent the primary zones of contaminant flux.
Exemplary prior art references reflecting the current state of technology includes the following publications, the teachings of each of which are expressly incorporated herein by reference:    Brown, M. J.; Burris, D. R. “Enhanced Organic Contaminant Sorption on Soil Treated with Cationic Surfactants. Groundwater 34(4), 1996, 734-744 (Brown).    Luthy, R.; Ghosh, U. U.S. Pat. No. 7,101,115 B2, 2006, “In situ stabilization of persistent hydrophobic organic contaminants in sediments using coal- and wood-derived carbon sorbents” (Luthy).    Mork, B.; Gravitt, J.; Ferguson, R.; Rittenhouse, S.; Thoreson, K. U.S. patent application Ser. No. 14/449,404, filed Aug. 1, 2014, “Colloidal agents for aquifer remediation” (Mork).
For example, a method for increasing the foc of an aquifer in the prior art is described by Brown where cationic surfactants that can be injected into the subsurface and adhere to the soil matrix are used in order to alter the native foc and increase the retardation of contaminants. While this does provide a temporary increase in the foc, this is not a long term solution as surfactants are degradable and therefore the increase in foc will not be permanent. This temporary increase is the limitation of any degradable substance that is capable of increasing the foc.
In another example, Luthy describes the use of activated carbon to reduce the flux of contaminants moving from the sediment to the water. While this method does provide a more permanent fix, it requires the activated carbon to be physically mixed into the aquifer (e.g. large auger mixing) or high pressure fracturing the subsurface which limits the widespread emplacement of the material, disrupts the natural flow channels of the aquifer, and can increase application costs.
Thus lacking in the art are methods for permanent contaminant containment using an agent that could be added to the subsurface as a means of increasing the retardation factor in a controlled method whereby the agent possesses the preferred characteristics that include: a solid material that can be permanently emplaced in the aquifer, a material that is not degradable and will last for a sufficiently long period of timed (i.e., decades) and is a material that has a high affinity for the contaminants of concern, especially hydrophobic contaminants. It would therefore be desirable to be able to add such an agent to the subsurface that provides the equivalent effect as increasing the foc in order to increase the retardation factor for COC plumes, especially in the primary or high flux zones. The ability to do this would result in improved management of plumes by preventing contaminants from migrating toward sensitive receptors and may sufficiently reduce the risk associated with these plumes to allow for environmental regulators to deem the plume-impacted sites fit for intended use. Additionally, it would be desirable to be able to emplace the desired solid material via injection under low (non-fracturing) pressures and have the material be in a form that is able to distribute widely in the aquifer under low injection pressures or natural groundwater flow conditions. In comparison to high pressure injection of agents or soil mixing, low pressure injections of a distributable reagent will reduce the emplacement costs, enhance the widespread distribution of the agent in the permeable channels to maximize effectiveness, and minimize the disturbance to the subsurface and the flux zones. An additional benefit of engineering the retardation factor of a migrating plume results in the ability to control the residence time of a contaminant within a reactive zone that promotes the biodegradation of biodegradable contaminants, thus further improving the ability to manage a contaminant plume.